J. C. van den Heuvel's research while affiliated with Delft University of Technology and other places

Publications (86)

Article
Space Charge Limited Current (SCLC) measurements are used to obtain the density of gap states of intrinsic a-Si:H. If an electron is trapped by a positively charged defect, then the electron can be released by a high electric field which disturbs the SCLC measurement. A correction for this effect, which is called Poole-Frenkel emission, in SCLC mea...
Article
In hydrogenated amorphous silicon NMR studies indicate a twophase structural inhomogenity. Hydrogenated microcrystalline films consist of small crystals with a typical size of 100 Å, embedded in an amorphous web. A hydrogen rf plasma is able to etch both type of films, but with different etch rates. The more crystalline parts of a film are etched m...
Article
Coastal surveillance and naval operations in the littoral both have to deal with the threat of small sea-surface targets. These targets have a low radar cross-section and a low velocity that makes them hard to detect by radar. Typical threats include jet skis, FIAC's, and speedboats. Previous lidar measurements at the coast of the Netherlands have...
Article
Full-text available
In recent years, much advance in the field of high-power femtosecond laser technology has been made. The high pulse power of femtosecond laser systems leads to various interesting phenomena, such as a very high power density and the formation of a plasma in the propagation medium, which is usually air. The possible application of femtosecond lasers...
Article
Current coastal operations have to deal with threats at short range in complex environments with both neutral and hostile targets. There is a need for fast identification, which is possible with a laser range profiler. A number of field trials have been conducted to validate the concept of identification with a laser range profile. A laser range pr...
Article
The reduced and reoxidized Cu-Zn-Si oxide catalysts as layered copper-zinc hydroxo silicates with the zincsilite structure were studied using in situ and ex situ X-ray diffraction analysis, transmission electron microscopy, and the temperature dependence of magnetic susceptibility. The catalysts were prepared by homogeneous deposition-precipitation...
Article
Naval operations in the littoral have to deal with the threat of small sea-surface targets. These targets have a low radar cross-section and low velocity, which makes them hard to detect by radar in the presence of sea clutter. Search lidars can provide an alternative detection capability for small sea-surface targets at ranges up to 10 km (using c...
Article
Naval operations in the littoral have to deal with the threat of small sea-surface targets. These targets have a low radar cross-section and low velocity, which makes them hard to detect by radar in the presence of sea clutter. Search lidars can provide an alternative detection capability for small sea-surface targets at ranges up to 10 km (using...
Article
Full-text available
Naval operations in the littoral have to deal with threats at short range in cluttered environments with both neutral and hostile targets. There is a need for fast identification, which is possible with a laser range profiler. Additionally, in a coastal-surveillance scenario a laser range profiler can be used for identification of small sea-surface...
Article
Full-text available
Coastal surveillance and naval operations in the littoral both have to deal with the threat of small sea-surface targets. These targets have a low radar cross-section and a low velocity that makes them hard to detect by radar. Typical threats include jet skis, FIAC's, and speedboats. Lidar measurements at the coast of the Netherlands have shown a v...
Conference Paper
Laser countermeasures against infrared focal plane array cameras aim to saturate the full camera image. In this paper we will discuss the results of dazzling experiments performed with MWIR lasers. In the "low energy" pulse regime we observe an increasing saturated area with increasing power. The size of the saturated area can be explained by an ex...
Conference Paper
Full-text available
We have developed a stereo vision based obstacle detection (OD) system that can be used to detect obstacles in off-road terrain during both day and night conditions. In order to acquire enough depth estimates for reliable OD during low visibility conditions, we propose a stereo disparity (depth) estimation approach that uses fine-to-coarse selectio...
Conference Paper
Full-text available
Autonomous robot vehicles that operate in off-road terrain should avoid obstacle hazards. In this paper we present a stereo vision based method that is able to cluster reconstructed terrain points into obstacles by evaluating their relative angles and distances. In our approach, constraints are enforced on these geometric properties by a set of pix...
Article
The composites ZnOCuH(D) saturated with hydrogen (deuterium) to a content of ∼1 wt % are investigated by the neutron scattering methods. Upon cooling of the samples (the ZnO matrix containing Cu crystals ∼10 nm in size) from 300 to 4 K, hydrogen (deuterium) is condensed on the cluster surface and penetrates inside the clusters in which the atomic...
Article
Naval operations in the littoral have to deal with threats at short range in cluttered environments with both neutral and hostile targets. On board naval vessels there is a need for fast identification, which is possible with a laser range profiler. Additionally, in a coast-surveillance scenario a laser range profiler can be used for identification...
Article
Naval operations in the littoral have to deal with the threat of small sea-surface targets. These targets have a low radar cross-section and low velocity, which makes them hard to detect by radar in the presence of sea clutter. Typical threats include periscopes, jet skies, FIAC's, and speedboats. Search lidars on board naval vessels can provide de...
Article
Full-text available
Laser countermeasures against infrared focal plane array cameras aim to saturate the full camera image. In this paper we will discuss the results of three different dazzling experiments performed with MWIR lasers and show that the obtained results are independent of the read-out mechanism of the camera and can be explained by an expression derived...
Article
The atomic and cluster structure of hydrogen-treated Cu/ZnO (the major component of methanol synthesis catalysts) was studied using Bragg powder and small-angle neutron diffraction. Isotopic substitution of Cu and H was used to obtain reliable results indicating Cu and H clustering. The clusters form on the boundaries of ZnO particles and have a co...
Article
Acoustic Landmine Detection (ALD) using a laser vibrometry was studied using two acoustic ground excitation methods: acoustic (loudspeaker) excitation (AE) and laser excitation (LE). AE is suitable for detecting buried landmines but is a slow technique. A faster AE method based on a CMOS camera was proposed but not studied further. LE is potentiall...
Conference Paper
Full-text available
Terrain classification is an important problem that still remains to be solved for off-road autonomous robot vehicle guidance. Often, obstacle detection systems are used which cannot distinguish between solid obstacles such as rocks or soft obstacles such as tall patches of grass. Terrain classification is needed to prevent that the robot is stoppe...
Article
Small maritime targets, e.g., periscope tubes, jet skies, swimmers and small boats, are potential threats for naval ships under many conditions, but are difficult to detect with current radar systems due to their limited radar cross section and the presence of sea clutter. On the other hand, applications of lidar systems have shown that the reflect...
Article
The structure of Cu/SiO 2 and Cu/ZnO/SiO 2 catalysts was studied after reduction at 450-1300 K. The influence of the ZnO promoter on the exposed Cu surface area and metal cluster size was determined by N 2 O chemisorption and X-ray diffraction. After reduction at 450 K, the metal surface area amounted to 9 m 2 /g cat for both catalysts. Oxygen upta...
Article
Countermeasures against heat seeking missiles require access to efficient laser sources, which should emit wavelengths at band I, II and IV. Efficient diode pumped solid-state lasers, combined with efficient non-linear wavelength shifters, allow the development of practical tuneable mid-IR countermeasure sources. The paper describes the requirement...
Article
Countermeasures against heat seeking missiles require access to efficient laser sources, which should emit wavelengths at band I, II and IV. Efficient diode pumped solid-state lasers, combined with efficient non-linear wavelength shifters, allow the development of practical tuneable mid-IR countermeasure sources. The paper describes the requirement...
Article
Acoustic landmine detection (ALD) is a technique for the detection of buried landmines including non-metal mines. Since it gives complementary results with GPR or metal detection, sensor fusion of these techniques with acoustic detection would give promising results. Two methods are used for the acoustic excitation of the soil: laser excitation and...
Article
Vehicles that serve in the role as landmine detection robots could be an important tool for demining former conflict areas. On the LOTUS platform for humanitarian demining, different sensors are used to detect a wide range of landmine types. Reliable and accurate detection depends on correctly combining the observations from the different sensors o...
Article
Full-text available
Vehicles that serve in the role as landmine detection robots could be an important tool for demining former conflict areas. On the LOTUS platform for humanitarian demining, different sensors are used to detect a wide range of landmine types. Reliable and accurate detection depends on correctly combining the observations from the different sensors o...
Article
Acoustic landmine detection (ALD) is a technique for the detection of buried landmines including non-metal mines. An important issue in ALD is the acoustic excitation of the soil. Laser excitation is promising for complete standoff detection using lasers for excitation and monitoring. Acoustic excitation is a more common technique that gives good r...
Article
Full-text available
We present a method, based on stereo vision, for estimating the position and orientation of the LOTUS platform and its sensors. The LOTUS platform was developed to demonstrate the capabilities of automated landmine detection. Motion of the platform and pose and position of its sensors have to be measured to relate different sensors observations acc...
Article
Oxygen limitation in solid-state fermentation (SSF) has been the topic of modeling studies, but thus far, there has been no experimental elucidation on oxygen-transfer limitation at the particle level. Therefore, intra-particle oxygen transfer was experimentally studied in cultures of Rhizopus oligosporus grown on the surface of solid, nutritionall...
Conference Paper
In this paper a stereo vision method is described developed for autonomous vehicle navigation in unstructured terrain. The disparity between the stereo image sequences is calculated with subpixel accuracy. Robust distance values are obtained by adding a continuity criterion term in the evaluation criterion and by using confidence measures for the f...
Conference Paper
This paper provides an overview of the RoboJeep unmanned vehicle and a control station for teleoperation. The RoboJeep is a testbed for the integration of key technologies for semi-autonomous vehicle operation. The RoboJeep has a layered control system architecture, allowing an incremental development of control layers. The control station enables...
Article
Airborne lidar signals of cirrus clouds are analyzed to determine the cloud structure. Climate modeling and numerical weather prediction benefit from accurate modeling of cirrus clouds. Airborne lidar measurements of the European Lidar in Space Technology Experiment (ELITE) campaign were analyzed by combining shots to obtain the backscatter at cons...
Article
Diffusion coefficients of actual metabolites in completely active biofilms can be determined by applying a new concept that is based on a constant local activity in the entire biofilm. In that case, a concentration step will be transmitted unattenuated. Subsequently, the diffusion coefficient can be calculated from the response monitored with a mic...
Article
Full-text available
The change of activity and abundance of Nitrosospira and Nitrospira spp. along a bulk water gradient in a nitrifying fluidized bed reactor was analyzed by a combination of microsensor measurements and fluorescence in situ hybridization. Nitrifying bacteria were immobilized in bacterial aggregates that remained in fixed positions within the reactor...
Article
High operational stability and productivity of co-immobilised systems are important aspects for their successful application in industrial processes. A dynamic model is required to describe artificially co-immobilised systems because the time needed to reach steady state normally exceeds the operational life span of these systems. Time dependent in...
Article
High operational stability and productivity of co-immobilised systems are important aspects for their successful application in industrial processes. A dynamic model is required to describe artificially co-immobilised systems because the time needed to reach steady state normally exceeds the operational life span of these systems. Time dependent in...
Article
The mobility of water in intact biofilms was measured with pulsed field gradient nuclear magnetic resonance (PFG-NMR) and used to characterise their diffusive properties. The results obtained with several well-defined systems, viz. pure water, agar, and agar containing inert particles or active bacteria were compared to glucose diffusion coefficien...
Article
A short-pulse 1.444-μm laser based on Nd:YAG technology has been demonstrated. The 1.444-μm is eye-safe. With the cavity-dump technique, a pulse of 50 m× and 14 ns was obtained. The beam quality was excellent with an M<sup>2</sup> of 1.6 by the use of a telescopic resonator. Silicon-window polarizers were used to suppress the 1.06-μm radiation but...
Article
The generation of 3 to 5 microns laser radiation and the required powers to jam IR-seekers were investigated in relation to a laser-jamming system. Sufficient powers were reached with two generation methods. The first method used difference frequency generation (DFG) to obtain 4.04 microns radiation using the mixing in LiNbO3 of 1.064 microns and 1...
Article
Gas bubbles entrapped in biocatalyst particles subjected to hydrostatic pressure oscillations, e.g. during recirculation in loop reactors, will induce intraparticle liquid flows, and thereby enhance mass transfer in excess of diffusion. This ‘breathing particle’ mechanism was already demonstrated in methanogenic granules from an IC reactor, and led...
Article
Gas bubbles entrapped in biocatalyst particles subjected to hydrostatic pressure oscillations, e.g. during recirculation in loop reactors, will induce intraparticle liquid flows, and thereby enhance mass transfer in excess of diffusion. This ‘breathing particle’ mechanism was demonstrated in methanogenic granules, and led to a typical activity incr...
Article
Gas bubbles entrapped in catalyst particles subjected to hydrostatic pressure oscillations, e.g. during recirculation in loop reactors, will induce intraparticle liquid flows, and thereby enhance mass transfer in excess of diffusion. This ‘breathing particle’ concept was clearly demonstrated in a gas-producing model system. At an amplitude of 0.3 M...
Article
One can use stimulated Raman scattering to obtain a near-diffraction-limited Stokes beam from a multimode pump beam if the Raman amplifier is seeded with a TEM00 Stokes beam. In this way a smooth beam is obtained from a rough beam; hence the term beam cleanup. We investigate the Raman amplifier configuration numerically, using various pump beam pro...
Article
Stokes beam quality as obtained by stimulated Raman scattering is investigated numerically using various pump-beam profiles. It is found that the quality of the Stokes beam, expressed by M<sup>2</sup>, is often roughly the same as the pump beam. For pump beams with Gaussian-Hermite modes, TEM<sub>00</sub> and TEM<sub>01</sub>, the Stokes beam has t...
Article
Pellets of Penicillium chrysogenum which were spontaneously formed after a certain stage of a batch fermentation, displayed a considerable structural change in course of their lifetime. Microelectrode studies showed the internal mass transport properties of these pellets (diameter 1–3 mm) to be highly effected by their morphological structure. Rela...
Article
The conversion efficiency of a Raman cell at high pump energies has been increased with an astigmatic focus. Experimental and numerical results show that the increased conversion is due to the reduction of cascade second order Stokes. For our experimental setup, it is shown that other effects, namely Brillouin scattering, anti-Stokes generation, an...
Article
An astigmatic focus has been used to increase the conversion efficiency of a Raman cell at high pump energy. Experimental and numerical results show that the increased conversion is due to the reduction of cascade second order Stokes. It is shown that other effects, namely Brillouin scattering, anti-Stokes generation, and ground-state depletion, ar...
Article
Full-text available
Microelectrodes for ammonium, oxygen, nitrate, and pH were used to study nitrifying aggregates grown in a fluidized-bed reactor. Local reactant fluxes and distribution of microbial activity could be determined from the microprofiles. The interfacial fluxes of the reactants closely reflected the stoichiometry of bacterial nitrification. Both ammoniu...
Article
Full-text available
Microelectrodes forammonium, oxygen,nitrate, andpH were usedtostudy nitrifying aggregates grown in afluidized-bed reactor. Local reactant fluxes anddistribution ofmicrobial activity couldbedetermined from themicroprofiles. Theinterfacial fluxes ofthereactants closely reflected thestoichiometry ofbacterial nitrification. Bothammoniumconsumption andn...
Article
The threshold power of stimulated Raman scattering (SRS) has been calculated for various nondiffraction-limited pump beams using a numerical model. These calculations were performed for the following pump beam profiles: Gaussian-Hermite, Gaussian-Laguerre, Gaussian-Schell-model (GSM), and several superpositions of Gaussian-Hermite modes. It is show...
Article
Due to the conversion of acetic acid into the weaker carbonic acid and methane, the pH inside methanogenic aggregates is higher than in the bulk liquid. The pH profiles in aggregates were measured with pH microelectrodes. These profiles strongly determine the macro-kinetics of the aggregate, by their influence on the values of the growth parameters...
Article
Experiments were performed to obtain a high efficiency for conversion of 1064 nm to eye-safe 1543 nm radiation by stimulated Raman scattering (SRS) in methane. A first order Stokes photon conversion efficiency of about 75 percent is obtained for an input energy of 280 mJ, using combination of cylindrical lenses for focusing. The highest obtainable...
Article
A two-dimensional finite difference method is presented for the propagation of beams focused by an astigmatic lens. The method is used for the modeling of a Raman amplifier that uses astigmatic lenses. The effect of gain focusing on the amplification is studied with this method. The numerical results show a good correspondence with analytical model...
Chapter
Biological conversions are characterized by relatively low specific reaction rates. Many processes in which micro-organisms play a role are feasable on an industrial scale due to their potential to form aggregates spontaneously or to attach themselves as thin biofilms on a carrier material. As a consequence of these adhesive properties, it is possi...
Article
Full-text available
Product inhibition of the acidogenic population from an anaerobic wastewater treatment plant was studied in a carefully controlled fed-batch culture. Repeated feeding with a concentrated glucose solution allowed rapid determination of the critical product concentration at which metabolism ceased completely. From the course of the different products...
Chapter
The use of immobilized cells is advocated for a number of bioengineering purposes, e.g. volumetric production rate, retention of expensive biomass, product quality, and convenient downstream processing. Up to now, succesfull application of such biofilms is rather empirical, since detailed knowledge of the internal structure and performance usually...
Article
Full-text available
Ammonium microprofiles in two dominant types of lake sediment were measured using ion-selective microelectrodes. These profiles enabled establishment of local fluxes and convensions. Comparison of ammonium profiles obtained from the analysis of sediment slices with those measured with microelectrodes showed the superior spatial resolution of the la...
Article
A glucose micro-electrode was developed for direct measurements inside biofilms, and applied for the determination of effective diffusion coefficients in a model system of agar beads containing immobilized yeast cells. Two methods were used, one based on concentration gradients present at the liquid/solid interface of an active biofilm under steady...
Article
The surface photovoltage (SPV) technique is often used to determine the diffusion length in hydrogenated amorphous silicon (a‐Si:H). The use of this technique for a‐Si:H is disputed because it has been shown by computer simulations that the drift component is not negligible compared to the diffusion current in SPV measurements on a‐Si:H. For crysta...
Article
Hydrogenated amorphous and microcrystalline silicon films were deposited using the RF glow discharge method. Silane-hydrogen mixtures were used. The conditions under which amorphous and microcrystalline silicon are formed have been investigated with regard to the deposition rate. The deposition rate of good quality amorphous and microcrystalline si...
Article
A new method for the analysis of diffusion length measurements by the Surface Photovoltage (SPV) method is presented. It takes into account the effect of the reflection of light from the back contact in thin layers and the effect of a finite bandwidth of the used interference filters. The model was found to agree with experiments on thin amorphous...
Article
An ammonium ion-selective microelectrode is described. With this microelectrode, product gradients were measured in an enzymic model system consisting of cross-linked urease immobilized in spherical agar gels. The measured and calculated gradients showed a good correspondence. pH gradients measured in a microbial system of aggregated methanogens di...
Article
Full-text available
Ammonium-selective microelectrodes made with an ion-exchanger based on nonactin and having a tip diameter of 1 mum have been developed. The response of the electrode is linear from 10(-5) to 10(-1)M NH(+)(4) and the response time is 1 min. Applications of these electrodes can be found in biotechnology and microbial ecology, but the low selectivity...
Article
Full-text available
In anaerobic wastewater treatment the separation of fermentative and methanogenic bacteria is aimed at an increased performance of the total digestion process. It is known that the attainable growth rate of the acidogenic population in continuous culture decreases at increasing influent concentrations of glucose. To account for this phenomenon, a n...
Article
The starting point for the present investigations was the finding that increasing influent concentrations from 10 to 380 mmol/L glucose decreased the attainable growth rate of an acidogenic population in continuous culture from 0.52 to 0.05 h(-1) To account for this phenomenon, a new kinetic model is developed that combines substrate and product in...
Article
Full-text available
A dilution-rate shift-up was employed to induce bacterial hold-up in a continuous-flow gas-lift reactor. A minimum carrier concentration (sand, 2–5 g/l) was found a prerequisite for formation of bacterial aggregates, which fermented glucose either to propionate/acetate or to butyrate/acetate. Higher levels of sand did not affect the onset of propio...
Article
The magnitude of metastable light-Induced degradation of intrinsic hydrogenated amorphous silicon films deposited by rf glow discharge from different selene-hydrogen mixtures wan studied. By stone of spacecharge-limited current measurements the Fermi-lava1 density of states was determined for as-grown and light-soaked n+-t-n+ devices. Films deposit...
Article
The sub-bandgap absorption of intrinsic hydrogenated amorphous silicon films deposited by rf glow-discharge from different silane-hydrogen mixtures has been measured by means of PDS. Using a mixture of 55 vol.% silane, 45 vol.% hydrogen and a relatively high rf power level of 36 mW/cm2 to obtain a high growth rate, yielded films with an improved st...
Article
A start-up routine for a novel type of anaerobic gas-lift reactor using sand as support particles for Bacterial adhesion and involving a dilution rate shift-up is shown to result in rapid formation of mixed-culture aggregates from freely suspended cells. Formation of aggregates changed the general metabolism from acetate-butyrate production to acet...
Article
The etching of amorphous and microcrystalline silicon films in a hydrogen plasma has been investigated. For amorphous silicon an etch rate of 2.4 A/s was found. Microcrystalline silicon is etched at a rate of 0.3 A/s. Microcrystalline silicon consists of two phases. The amorphous part is removed preferably.
Chapter
The effect of silane dilution with hydrogen at different power levels on the growth rate of intrinsic hydrogenated amorphous and microcrystalline silicon films has been studied. The results are explained from the point of chemical equilibrium. Good quality microcrystalline and amorphous layers are grown with nearly the same deposition rate. Above a...
Chapter
From the space limited current regime of the current-voltage characteristics of p-i-p and p-π5-p structures, the density of gap states was determined. Metastable changes in these curves were observed after current and light soaking. The current induced changes annealed at 100°C. The samples with i and layers of 0.5 – 1 µm thick, were all grown unde...
Article
In a CFSTR, inoculated with activated sludge, the acidogenic dissimilation at 30°C of a 1% glucose containing medium (C-limited), the maximum specific growth rate and product distribution, was studied over the pH range from 4.5 to 7.9. An exact description in terms of carbon mass- and O/R-balances is presented. The gas phase consisted of carbon dio...
Article
The waste water from a sugar refinery, containing principally sucrose, lactate and ethanol as organic impurities, was anaerobically acidified at a pH of approximately 5·8 and an average temperature of 29°C in a 1 m3 upflow reactor. The residence time was decreased, in steps, from 7·2 to 1·7 h. Sucrose was effectively metabolised in all cases, where...